The present invention relates to a device for outputting an electron plasma, and, in particular, a cold cathode operable at a significantly lower voltage and field strength.
The emission of electrons from the surface of a conductor into a vacuum or into an insulator under the influence of a strong electric filed has found many useful applications. One such application includes field emission microscopy in which some of the most powerful microscopes known have been constructed. Such microscopes generally utilize a "hairpin" cathode with a fine tungsten point at the apex of the hairpin as a source of electrons. Since the degree of magnification obtained by field emission microscopes is a function of the emission levels from the tungsten tip, it is desirable to utilize a hairpin filament with high emission levels so that high magnification can be obtained. Conditions conducive to high emission are a high operating temperature, an ultrahigh vacuum, and a high electric field. With these conditions, a relatively high emission can be obtained; however, the useful life of a hairpin filament operated in this manner is considerably reduced. Additionally, as a result of the high temperatures, the field emission microscope is limited in application to an investigation of those metals having a melting point higher than the operating temperature of the filament. Another application is the field of high power vacuum tube technology.
The conventional material used as a thermionic electron emission cathode for producing a shaped beam is tungsten. Lanthanum hexaboride (LaB.sub.6) has been used to produce an unshaped or round beam in the past because it has a lower work function, higher melting temperature, and a lower vapor pressure than tungsten. Thus, LaB.sub.6 cathodes have promised higher brightness at the same operating temperature and pressure, and longer life.
LaB.sub.6 has not been used to produce a shaped beam because the beams produced by LaB.sub.6 cathodes heretofore have always been characterized by a rather narrow angular distribution which is gaussian in shape. As a result, when such a beam is shaped with an aperture, the resulting beam does not have a uniform intensity distribution unless it is so small in size that it is impractical for use in microelectronic fabrication tools.
A tungsten cathode, on the other hand produces a very broad angular distribution and can generate an electron beam with very high total current. Although the tungsten produced beam is also gaussian, the angular distribution is so wide and the total beam so intense that a small center region of the angular distribution can be selected by a shaped aperture and the resulting beam has a nearly uniform intensity distribution.
Advanced cold cathode emitters currently in use employ low work function metals and alloys. These are formed by a variety of bulk solidification techniques. Unfortunately, these processing techniques dramatically reduce the operating efficiency and lifetime of these structures.
Bulk solidification techniques, such a eutectic solidification of LaB.sub.6, form material much closer to thermodynamic equilibrium. Hence the solid will exhibit properties that are a function of that chemical equilibrium condition.
Currently, very high power Klystron tubes have average lifetimes of less than 100 hours. The major failure mechanism can be attributed to overheating at high electron fluences.
The above operating conditions and material characteristic have created a need for an improved cold cathode.